Delete generated Bitbucket Server archives from server after download completes

When running a migration from Bitbucket Server, we ask the Bitbucket Server instance to generate an archive using its REST API, and the archive is stored on the server's filesystem.

We use SSH or SMB to download that file to the machine running the CLI - but the file is still left on the Bitbucket Server, taking up space and potentially being accessible by someone else.

Once we have downloaded the file, we should automatically delete it. Unlike in #818, I don't see a good reason to have an option to keep the file.